1. Field of the Invention
The present invention relates to an optical recording medium for recording and reproducing information, by use of laser beams, and a manufacturing method thereof, and, more particularly, to an optical recording medium having a plurality of information recording layers (hereafter, referred to as “recording layers”), and a manufacturing method thereof.
2. Description of the Related Art
In recent years, data handled by an information processing apparatus, such as a computer or a video apparatus, has diversified into various types of information, such as sound, image, and moving image, whereby various types of recording modes and information recording media have been utilized. The required data sizes of the information recording media have been increasing, and the demand for high density has been raised.
Optical disks record and reproduce information with a laser beam focused at a small region on an information recording surface. Any type of optical disk is of excellent high density, detachable (removable), and low-bit-cost, and is utilized in diverse applications, such as data distribution and information backup. Optical disks are roughly classified into a reproduction-only type, a write-once type, and a rewritable type, based on the structure of a recording layer.
Basically, the recording capacity of an optical disk is proportional to the recording density and the area of the information recording surface. The recording density is in inverse proportion to the size of a focused laser spot. A laser-spot diameter is determined by γ/NA (γ and NA denote a laser wavelength and a numerical aperture, respectively). Therefore, it is conceivable that, in order to increase the recording capacity, the information area is increased by utilizing a short-wavelength laser beam and by enlarging the numerical aperture of the lens. However, shortening a laser beam wavelength is restricted by the elements and enlarging the diameter of a disk, so that an increase in the information area makes the handling of the disk difficult. Thus, enlarging a numerical aperture and providing a multilayered recording layer so as to increase the information area are main research themes.
With an increase in the numerical aperture of a lens, it is required to shorten the distance between the lens and the information recording surface. Therefore, the structure of an optical disk has been changed.
In the case of a CD (compact disk), an information recording surface is provided on a substrate of 1.2 mm in thickness, and a laser beam is radiated from the substrate side through the substrate, to achieve recording and reproduction.
In a DVD (digital versatile disk), recording and reproducing have been accomplished using the substrate in a similar way. However, the thickness of the substrate, on which the information recording surface is provided, has been reduced to 0.6 mm, to bring the lens close to the information recording surface. In order to reinforce the disk, a dummy substrate of 0.6 mm in thickness is attached to the surface opposite to the laser incident face of the information recording surface.
In the case of a BD (Blue-ray Disc), in which high density has been further achieved, for example, as disclosed in Japanese Patent Application Publication No. 2002-260307, a plurality of recording layers are laminated on a 1.1. mm-thick resin substrate, which is a base for an optical disk, and a protection layer of approximately 0.1 mm in thickness is provided on the recording layer. A laser beam is radiated from the protection-layer side so as to shorten the distance between the optical system and the information recording surface, thereby enlarging the lens numerical aperture. A structure is employed in which the recording layers are separated from one another, by respective intermediate layers, and the laser beam focused on a specific recording layer does not affect other recording layers.
A type of DVDE, which uses two-layered information recording films, has been put to practical use. Two methods of manufacturing the DVDs have been known. In one of the methods, which is known as “combining”, two 0.6 mm-thick substrates having a recording layer thereon are bonded together with an intermediate layer in-between. In the other method, a first recording layer, an intermediate layer, and a second recording layer are formed on a substrate of 0.6 mm in thickness, and a dummy substrate is finally bonded to the second recording layer. The combining is effective as a manufacturing method of a two-layer type DVD. However, the combining is not easily applied to a BD, because the protection layer in the BD is thin and difficult to handle. In bonding a dummy substrate to the second recording layer, a transmissive stamper, used to form track grooves on the intermediate layer, can be diverted to the dummy substrate. This has not been a cost factor. However, this method cannot be applied to a BD that does not have a dummy substrate.
A conventional technique will be further explained, by exemplifying a typical manufacturing method for a rewritable type BD having two recording layers.
FIGS. 6A, 6B, 6C, 6D, 6E and 6F are views schematically illustrating an example of a BD manufacturing method according to a conventional technique.
In the first place, a first recording layer 210 is formed on the main surface of a substrate 110 molded by use of a typical resin molding machine, on which information-tracking guide grooves (hereafter, referred to as “tracking grooves”) are formed (in FIG. 6A). In the case when the first recording layer 210 is a phase-change type, a typical sputtering apparatus can be utilized.
Next, an ultraviolet curing resin, which becomes an intermediate layer, is dispensed through a nozzle 700 on the first recording layer 210, which is rotating, and an intermediate layer 310 is spin-coated up to approximately 5 to 40 μm in thickness (in FIG. 6B).
A transmissive stamper 120 is closely contacted to the formed intermediate layer 310, and then, ultraviolet rays 900 are radiated onto the intermediate layer 310, so as to cure the intermediate layer 310 (in FIG. 6C). After the intermediate layer 310 is cured, the transmissive stamper is removed, so that tracking grooves for a second recording layer are formed on the top surface of the intermediate layer 310 (in FIG. 6D). The transmissive stamper 120 is discarded.
A second recording layer 220 is formed on the top surface of the intermediate layer 310, in which the tracking grooves have been formed (in FIG. 6E).
A protection layer 400, of 50 to 200 μm in thickness, is formed on the top face of the second recording layer 220, so that a BD is completed (in FIG. 6F). The protection layer 400 may be formed either by means of spin coating or by adhering a sheet-like member.
It is conceivable that, in order to make the transmissive stamper 120 durable to radiated ultraviolet rays, the stamper 120 is made of glass. However, it is difficult to remove the glass made stamper after the intermediate layer has cured. Thus, a resin material, which has a superior ability to separate, is generally utilized as a material for the transmissive stamper 120. However, radiation of ultraviolet rays makes a resin material change, and reduces the transmittance of the resin material. For that reason, a resin-made transmissive stamper is not recurrently utilized, and is discarded (in FIG. 6D), thereby increasing the production costs for a disk.